Les granites varisques du Massif Armoricain - HAL - INSU
Les granites varisques du Massif Armoricain - HAL - INSU
Les granites varisques du Massif Armoricain - HAL - INSU
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Zone Sud-<strong>Armoricain</strong>e, renferment les mêmes sources fertiles<br />
métasédimentaires à l'origine de certains <strong>granites</strong> <strong>varisques</strong>. Au contraire,<br />
dans chacune de ces zones les roches basiques et intermédiaires sont<br />
spécifiques et proviennent de source mantelliques différentes. Tout se passe<br />
comme si pour chaque zone, la croûte continentale comportait des<br />
caractéristiques communes, alors que le manteau lithosphérique serait<br />
différent.<br />
MOTS CLES : CHAINE VARISQUE, BRETAGNE, PLUTONISME, GRANITES, ASSOCIATIONS<br />
MAGMATIQUES, MAGMATISME JUVENILE, RECYCLAGE.<br />
ABSTRACT. A voluminous intrusive magmatism was emplaced between the end of the<br />
Devonian and beginning of the Permian in the Variscan belt of the Armorican<br />
<strong>Massif</strong>. We recognize here 5 plutonic suites, different in nature and origin,<br />
using new petro-geochemical criteria.<br />
1) An orogenic calc-alkaline suite highly variable in composition, from<br />
gabbros to biotite-hornblende <strong>granites</strong>, pro<strong>du</strong>ced by partial melting of an<br />
enriched mantle source above a sub<strong>du</strong>ction zone.<br />
2) A magneso-potassic, calc-alkaline and metaluminous suite, composed of<br />
(a) mafic to intermediate rocks, and (b) biotite ± hornblende porphyritic<br />
monzo<strong>granites</strong>. Both types could be totally mantle- derived but contamination<br />
and mixing processes with crustal <strong>granites</strong> are likely.<br />
3) A Fe-K alkaline to calc-alcaline and metaluminous suite composed of (a)<br />
mafic to intermediate rocks and (b) mainly of biotite ± hornblende<br />
monzo<strong>granites</strong> and syenites. According to isotopes, the mafic rocks are<br />
contaminated mantle rocks and felsic rocks are I-type crustal <strong>granites</strong> mixed<br />
with mantle material.<br />
4) A magneso-potassic, calc-alcaline and strongly peraluminous suite is<br />
composed of (a) mantle derived monzodiorites and (b) monzo<strong>granites</strong> and<br />
biotite-cordierite <strong>granites</strong>, mainly pro<strong>du</strong>ced from partial melting of metasedimentary<br />
sources with a high rate of melting. We observe frequent mixing<br />
between monzodiorites and <strong>granites</strong>.<br />
5) A leucocratic magneso-potassic, calc-alcaline and strongly peraluminous<br />
suite consists of muscovite-biotite leuco<strong>granites</strong> with various texture and<br />
modal composition. They were pro<strong>du</strong>ced by a low partial melting rate of<br />
metasediments, probably in relation with thermal relaxation processes. Raremetal<br />
<strong>granites</strong> are related to this suite.<br />
Mafic to intermediate rocks are found in four of the five magmatic suites. A<br />
juvenile mantle input clearly occured <strong>du</strong>ring crustal growth processes of the<br />
Variscan belt, but to mostly the Variscan Armorican plutonism derives from<br />
metasedimentary sources at different levels in the continental crust and from<br />
igneous sources in its lower part.<br />
It is interesting to notice that these Armorican intrusive rocks mostly consist<br />
of magmas of different nature and origin emplaced at the same time in a<br />
single massif.<br />
Finally, the distribution of the various suites gives some indications about<br />
composition of the lithosphere. The three main domains of the Armorican<br />
<strong>Massif</strong> (north, central and south) all contain similar metasedimentary fertile<br />
sources from which formed some Variscan <strong>granites</strong>. In contrast, in each of<br />
these domains, mafic and intermediate rocks are specific and derived from<br />
different mantle sources. It seems that in each domain, the continental crust<br />
was similar whereas the lithospheric mantle was different.<br />
KEY WORDS : Variscan belt, Brittany, plutonism, <strong>granites</strong>, magmatic suites, juvenile<br />
magmatism, recycling<br />
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